added stream seek test
added stream seek test

PyAV-Org/PyAV,danielballan/PyAV,mcpv/PyAV,pupil-labs/PyAV,markreidvfx/PyAV,markreidvfx/PyAV,pupil-labs/PyAV,xxr3376/PyAV,markreidvfx/PyAV,mcpv/PyAV,mikeboers/PyAV,pupil-labs/PyAV,PyAV-Org/PyAV,xxr3376/PyAV,mikeboers/PyAV,pupil-labs/PyAV,danielballan/PyAV,danielballan/PyAV,mcpv/PyAV,xxr3376/PyAV